Adickes was a critical empiricist, or moderate Kantian, and he wrote important works on Kant, including in 1895 a Kantian bibliography in German.

An opponent of Ernst Haeckel, Adickes in his Kant contra Haeckel wrote

I have no more belief than he in a personal extra-mundane God, a creation of the world by him, or an immaterial soul separated from the body.

He is also known for introducing in 1907, his concept of Four World-Views: Dogmatic (or Doctrinaire), Agnostic (or skeptical), Traditional, and Innovative, which would help shape personality theory in the 20th century.